The control of nanoparticle dispersability is important for many applications, such as size-selective nanoparticle precipitation, thin film deposition, and nanoparticle composite formation. Current methods for the preparation of monodisperse nanoparticles are costly, require a large quantity of solvent, or have a low throughput [1-3]. Surfaces of nanoparticles are often passivated with stabilizing ligands which prevent agglomeration by introducing thermodynamic repulsive forces. Understanding the forces between such ligands can provide insight into a better way to control nanoparticle dispersion . Recently, modelling efforts have shown that traditional steric repulsion and van der Waals attractions are not sufficient to characterize the interactions between nanoparticles in poor solvents . In this study, the importance of solvation forces on regulating the interactions of dodecanethiol ligands were quantified via atomic force microscopy (AFM). We chose toluene, hexane, and ethanol to represent good, mild and bad solvents, respectively to elucidate the forces involved in the interactions of dodeceanthiol (C12SH) ligands.
Materials and Methods:
Silicon nitride (Si3N4) AFM probes were coated with a 5 nm chromium adhesive layer followed by a 40 nm gold layer. The gold-coated cantilevers were then cleaned in absolute methanol and ethanol solution for 15 minutes each, respectively. Cantilevers were then immersed for 18 hours at room temperature in 1 mM ethanolic solution of 1-dodecanethiol for self-assembled monolayer (SAM) formation. Functionalized AFM cantilevers and substrates were then loaded into the AFM and imaging and force measurements were conducted in ethanol, hexane, and toluene. Areas of 2×2 μm2 were scanned at a rate of 1 Hz. Force-distance curves were collected with a ramp size of 1 μm at 1 Hz and with a 5 nN threshold force. Jump into contact and adhesion forces in the approach and retraction curves were analyzed, respectively.
Results and Discussion:
As the ligand-decorated AFM cantilever approaches or retracts from the opposing ligand surface, the ligands interact with each other. Representative approach and retraction force-distance curves are shown in Figure 1. As the ligand-decorated cantilever moves away from the opposing ligand surface, adhesion forces can be measured. The mean adhesion force was highest in ethanol (0.172 ± 0.073 nN) followed by toluene then hexane with adhesion forces of 0.165 ± 0.100 nN and 0.147 ± 0.063 nN, respectively. The high adhesion force between the hydrophobic C12SH layers in ethanol is largely due to the unwillingness of hydrophobic molecules to interact with hydrophilic solvents. Because ethanol is hydrophilic, the C12SH will want to interact with other C12SH molecules and will therefore not want to be separated. This increases the adhesion force or force required to separate the two C12SH SAM layers. In comparison to hexane, hydrophobic SAMs interacted with a higher force in toluene. This may be due to the π bonds that form between toluene and the extended C12SH polymers. In hexane, SAMs had the lowest retraction adhesion forces. This is mainly because hexane is hydrophobic and C12SH will likely interact with the solvent just as much as it will interact with the opposing C12SH layer.
In the approach curve or as the SAM layers come together, there is a jump into contact adhesion force that was analyzed with respect to its magnitude, frequency, and jump distance. The magnitude of these adhesion forces were 0.05 ± 0.02 nN for hexane, 0.08 ± 0.03 nN for ethanol, and 0.10 ± 0.03 nN for toluene. Although toluene seems to have the strongest adhesion force, these events only made up 11% of the curves whereas in hexane and ethanol, they were observed in 37 % and 60% of the registered approach curves, respectively. Since toluene is expected to extend the ligands, steric repulsion between the ligands will increase. Such increase in the repulsive forces hinder the attractive forces between ligands and thus reduce the probability of the occurrences of attractive forces. Ligands in hexane interacted with each other intermediately with the lowest strength. This is because the ligands are able to interact with the solvent as both are hydrophobic. However, in the presence of ethanol (hydrophilic solvent), the hydrophobic interactions will dominate the affinity of ligands and thus minimize the steric repulsion between the ligands.
The molecules interacted together in toluene at the longest distance (5.04 ± 3.58 nm). In comparison, molecules needed to be closer to each other for the jump into contact interactions to occur in ethanol and hexane with distances of 2.78 ± 1.11 nm and 2.90 ± 1.48 nm, respectively. These later distances were not statistically significantly different. The longer distance registered in toluene suggests that ligands adopt an extended conformation. It is important to note that jump distance does not represent the actual length of the ligands but the distance where the ligands start to attract each other. Currently, approach data is being fit to a model of steric repulsion to quantify the conformational properties of the SAM chains. All adhesion force and jump into contact data were statistically different among solvents tested. With respect to jump distance, toluene was statistically significantly different from the other treatment groups.
When in hydrophobic or good solvents, the C12SH ligands were extended whereas hydrophilic or bad solvents caused the ligands to collapse. Ligands interacted more favorably with each other when in hydrophilic solvents due to solvation excluding effects and vice versa when they were in good hydrophobic solvents. As a result, the use of hydrophilic solvents may be detrimental to dispersing nanoparticles.
Acknowledgements: This work was supported by the, NIH Protein Biotechnology Training Program 24280305, a NASA Space Grant, a WSU DRADS fellowship, and a Harold P. Curtis Scholarship.
Figure 1. Representative force-distance curves of DDT SAMs in various solvents. a) Retraction curves with the adhesion forces as quantified form the adhesion peaks b) Approach curves with jump into contact peaks. The jump distances were quantified as the difference between the distance of maximum peak value and the distance where there is no more attractive forces between ligands.
1. Murray, C.B., C.R. Kagan, and M.G. Bawendi, Synthesis and characterization of monodisperse nanocrystals and close-packed nanocrystal assemblies. Annual Review of Materials Science, 2000. 30: p. 545-610.
2. Murray, C.B., D.J. Norris, and M.G. Bawendi, Synthesis and characterization of nearly monodisperse CdE (E = sulfur, selenium, tellurium) semiconductor nanocrystallites. Journal of the American Chemical Society, 1993. 115(19): p. 8706-8715.
4. Saunders, S.R., M.R. Eden, and C.B. Roberts, Modeling the Precipitation of Polydisperse Nanoparticles Using a Total Interaction Energy Model. The Journal of Physical Chemistry C, 2011. 115(11): p. 4603-4610.